1. Field of the Invention
This invention relates generally to printed circuit boards in which micro-strips are provided to constitute respective signal lines of constant impedance for use in transmitting digital signals at high speed, for example, in a video switcher.
2. Description of the Prior Art
Known devices, such as, video switchers that may be employed in video special effects generators, generally use a coaxial cable to transmit a high frequency signal at high speed because a coaxial cable is characterized by a constant impedance required for transmitting a high frequency signal with high efficiency and high fidelity. Further, the coaxial cable is desirable in that is protects the high frequency signal being transmitted from being affected by extraneous disturbances or signals. However, when a number of signal lines are to be interconnected within a device, for example, a video switcher or the like, if coaxial cables are employed as constant impedance signal lines therefor, desirable miniaturization of the device cannot be effected and the cost of interconnecting the signal lines is relatively high.
On the other hand, if micro-strips of constant impedance on a printed circuit board are used as signal lines for transmitting respective high frequency signals at high speed, for example, as in a video switcher, such signal lines can be provided inexpensively and the device can be readily miniaturized as compared with the case where the signal lines are constituted by coaxial cables.
Therefore, as shown in FIG. 1, it has been proposed to provide a so-called double-sided printed circuit board 10 comprised of a dielectric lamina or sheet 11, for example, of a glass epoxy of the type known as FR-4, having opposed surfaces S.sub.1 and S.sub.2 at the front and back, respectively, of the dielectric lamina 11. The printed circuit board 10 is further shown to include a pattern 12 of signal lines of constant impedance constituted by respective spaced apart micro-strips suitably deposited on the front surface S.sub.1 and a ground plane or area 13 disposed on the surface S.sub.2 at the back of the dielectric lamina 11.
The characteristic impedance Z (in ohms) of the printed circuit board 10 may be expressed by the following equation: ##EQU1## where W is the width of each micro-strip included in the pattern 12 of signal lines formed on the surface S.sub.1, t is the thickness of each micro-strip, h is the thickness or height of the dielectric lamina 11, that is, the distance from each micro-strip or signal line 12 on the surface S.sub.1 to the ground plane or area 13 on the surface S.sub.2 of the lamina 11, and .epsilon. is the dielectric constant of the material making up the dielectric lamina 11 and serving as the isolating material of the printed circuit board 10.
In the case of a double-sided board, as shown in FIG. 1, the pattern 12 of one or more signal lines is formed only on the front surface S.sub.1 so that the double-sided board 10 is substantially equivalent to a single layer board or relatively low density or signal handling efficiency. If other printed circuits are provided on the board 10 along with the signal line pattern 12, the density of such single pattern of signal lines on the board will not be sufficient to accommodate the transmission of signals to and from the other circuits which are provided on the same board. Therefore, as shown in FIG. 2, it has been proposed to provide a so-called four-layer board 20 comprised of a first dielectric lamina 21 having opposed surfaces S.sub.1 and S.sub.2, a first layer L.sub.1 deposited on the surface L.sub.1 and comprised of a pattern 24 of micro-strips constituting one or more signal lines, and a second layer L.sub.2 deposited on the surface S.sub.2 of the dielectric lamina 21 and constituting a ground plane or area. The printed circuit board 20 is further shown to include a second dielectric lamina 22 having opposed surfaces S.sub.3 and S.sub.4 on which there are respectively deposited a third layer L.sub.3 comprised of a pattern 25 of micro-strips constituting one or more signal lines, and a fourth layer L.sub.4 which constitutes a ground plane or area. Finally, the printed circuit board 20 is shown to include a third dielectric lamina 23 having opposed surfaces S.sub.5 and S.sub.6 and being interposed between the first and second dielectric laminae 21 and 22 with the second layer L.sub.2 on the lamina 21, that is, the ground area or plane on the surface S.sub.2, and the fourth layer L.sub.4 on the lamina 22, that is, the ground area or plane on the surface S.sub.4, facing toward the opposed surfaces S.sub.5 and S.sub.6, respectively, of the third dielectric lamina 23. It will be appreciated that, in the printed circuit board 20, the layers L.sub.1 and L.sub.3 provide signal lines at the opposite outer surfaces S.sub.1 and S.sub.3 of the circuit board. Since the four-layer board 20 shown in FIG. 2 includes ground planes or areas constituted by the layers L.sub.2 and L.sub.4 interposed between the layers L.sub.1 and L.sub.3 which constitute the patterns 24 and 25 of signal lines on the front and back surfaces S.sub.1 and S.sub.3, respectively, of the circuit board, signals transmitted through the signal lines constituted by the layer L.sub.1 will not be affected or disturbed by signals transmitted through signal lines constituted by the layer L.sub.3.
However, if the micro-strips 12 deposited only on the surface S.sub.1 in the case of the printed circuit board 10 are arranged so as to form two or more independent signal lines, as indicated at 12a, 12b and 12c on FIG. 1, and different signals are transmitted through such lines 12a, 12b and 12c, then cross-talk will occur between the adjacent lines on the surface S.sub.1. Similarly, in the case of the four-layer printed circuit board 20 shown in FIG. 2, if the micro-strips forming the layer L.sub.1 are arranged to constitute independent signal lines indicated at 24a, 24b and 24c, then cross-talk will occur between the adjacent signal lines represented at 24a, 24b and 24c on surface S.sub.1. In the same manner, if the micro-strips constituted by the layer L.sub.3 are arranged to form independent signal lines indicated at 25a, 25b and 25c, then cross-talk will occur between the signals respectively transmitted through the adjacent signal lines 25a, 25b and 25c. Such cross-talk between signals transmitted through independent signal lines constituted by adjacent micro-strips on the surface S.sub.1 or S.sub.3 will cause deterioration of the signal transmission efficiency as compared with the case in which signals are transmitted through respective coaxial cables.
In the case of standard printed circuit boards of either the double-sided type (FIG. 1) or the multi-layer type (FIG. 2), but which are not fabricated with the signal lines on one or more of the surfaces thereof, as at 12a-12c on FIG. 1 or at 24a-24c and 25a-25c on FIG. 2, it has been known to provide the printed circuit board with a hole extending therethrough for receiving a screw or other fastening element by which the printed circuit board is attached to the device employing the same. For example, FIG. 3 shows a standard double-sided printed circuit board 10' comprised of a dielectric lamina 11' having a hole 26 board therethrough for receiving an attachment screw (not shown) which also serves to electrically connect ground planes 12' and 13' deposited on the front and back surface S'.sub.1 and S'.sub.2 of the lamina L'. However, it is to be understood that the standard printed circuit board 10' typified by FIG. 3 does not include signal lines constituted by micro-strips on a surface of the dielectric lamina 11'.